It is well known in the art to use light transmitted through or reflected from a medium in order to determine characteristics of the medium. For example, in the medical field, where non-invasive physiological monitoring of vital signs of a patient is often required, light transmitted through a portion of the body, and reflected or scattered from the body surface may be measured to determine information about the patient.
For example, during surgery, blood pressure, heart rate, breathing rate and blood oxygen saturation are often monitored. Moreover, for some individuals, there may be a daily, even hourly need to measure such parameters to know the individuals health and/or to detect and treat some diseases.
Furthermore, information about vital signs can also be important to individuals involved in athletic training and physical exercising. For example, one of the important applications related to physical activity is continuous heart rate monitoring. This field still requires developments in a sense that a suppressive majority of nowadays optical sensors performing heart rate monitoring must be attached to body parts, which is inconvenient as well as relatively unreliable, mainly due to the dependency on motion-artifacts. Other kinds of related applications are related to blood pressure monitoring, oximetry, breathing rate monitoring, etc. Accordingly, the most common requirement for all of the corresponding monitoring devices is the ability to be stable, compact, sensitive and reliable under operation with batteries.
A number of optical monitoring techniques have been proposed in the art that use light as an optical signal transmitted through a medium, such as a portion of a blood perfused body tissue with the goal of determining vital signs. Generally, such a monitoring system (also known as a photoplethysmograph) includes a transmitter utilizing a probe clipped on a part of the body (e.g., a finger, forehead, ear pinna or an earlobe) that includes an optical source, e.g., a light emitting diode (LED) or a laser, for irradiating the body part with light placed on one side of the of the body part while a photodetector is placed on an opposite side of the body part.
The monitoring system also includes a receiver utilizing an optical photodetector (e.g., a photo diode) positioned in an optical path so that it has a field of view which ensures the capture of a portion of the light which is transmitted, reflected or scattered from the body part. The optical detector converts the light (i.e., optical signal) into an analog electrical signal, which is subsequently amplified and provided to an analyzer to retrieve information that was present in the optical signal.
An example of the medical monitoring device using light transmitted through a portion of the blood perfused body tissue is a pulse oximeter. Pulse oximetry is used to determine the oxygen saturation of arterial blood. Oxyhemoglobin mainly absorbs infrared light while deoxyhemoglobin mainly absorbs visible red light. Accordingly, pulse oximeter devices typically contain two types of light sources, either light emitting diodes or laser diodes, operating in the red band of light and in the infrared band of light, respectively. Pulse oximeter devices also include photo-detectors for each of above mentioned wavebands and the processing unit that detects the ratio of red/infrared absorption and calculates the patient's oxygen saturation of arterial blood.
Specifically, transmission of optical energy as it passes through the body is strongly dependent on the thickness of the material through which the light passes, or the optical path length. Many portions of a patient's body are typically soft and compressible. Therefore, when the patient moves, the thickness of material through which optical energy passes can change. This results in the changes of the optical path length. For example, if optical energy passes through a finger and the user of an optical device moves in a manner which distorts or compresses the finger, the optical path length changes. Changes in the optical path length together with the changes of venous blood movement through during motion can produce enough distortion in the measured signal to make it difficult or impossible to determine desired information.
For example, U.S. Pat. Appl. Pub. No. 2009/0227853 describes an ear hook plethysmography (PPG) sensor and/or pulse oximetry (SpO2) sensor that can be attached to the skin in the regions of superficial artery and vein and posterior auricular artery and vein around the ear. For example, an ear wearable heart rate monitor can be constructed with these sensors.
U.S. Pat. No. 5,551,423 describes a pulse oximeter probe in the form of a clip that can be attached to an earlobe. The probe includes a pair of holding members that can be connected together at an end in such a way that they can pivot on a shaft. The holding members are arranged with a light-emitting device and a light-receiving device, in such a way that they are in a face-to-face relationship. The shaft is fitted with a leaf spring that urges the light-emitting and light-receiving devices to pivot in a direction in which they approach each other. The probe can be attached to an earlobe of a subject by holding the earlobe with the holding members.
It was noted that such an oximeter probe of the clip type has two major drawbacks. First, the holding members have to compress the earlobe so as to detect the pulsation of blood flowing in the compressed area but, then, the quantity of blood circulation decreases to lower the precision of measurement. Second, the probe which is attached to the earlobe is liable to movements and, hence, errors due to the movement of the earlobe are most likely to occur if measurements are done while the subject is walking.
To avoid these drawbacks, U.S. Pat. No. 5,551,423 providing a clip pulse oximeter probe that can be attached to the ear of a subject without compressing the site of measurement, and that is less sensitive to unwanted movements of the neck. The probe includes a pair of holding members pivotable on a shaft and configured for holding the basal part of the earlobe of a subject. The probe also includes a measuring section that consists of the light-emitting and light-receiving elements which are provided on the respective holding members in a face-to-face relationship. The compressing portions which hold the basal part of the earlobe are separated in position from the measuring section. One of the two holding members forms a bent portion at an end that can be inserted into the entrance to the auditory meatus, whereby the probe can be securely attached to the ear. In operation, the pulse oximeter probe detects the pulsation of blood in a blood vessel by reception of light at a light-receiving element after it is transmitted through a part of the earlobe.